Why do I need to choose the SIM card to use when sending a text?

[Shankbone]

I have just purchased a OnePlus 2that has dual SIM card slots, but I only have one (like most people). Every time I send a message it pops up a choice of which SIM to use (even though I only have 1 present). Is it possible to turn that off, either in the app or on the phone?